Method of coating elongated articles by immersion



NOV. 10, 19?@ HUNTER El'AL 3,539,385

METHOD OF COATING ELONGATED ARTICLES BY IMMERSION Filed July 29, 1968 E 1 =11: 11mm (IRS 4 OESQDKE' Wanda" c m aux/2mm 751 fffd/cr ATTORNEY United States Patent M 3,539,385 METHOD OF COATING ELONGATED ARTICLES BY IMMERSION Joseph E. Hunter, Rochester, and William K. Miller,

Southfield, Mich., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed July 29, 1968, Ser. No. 748,437 Int. Cl. 1305c 3/12, 11/00; C23c 1/06 US. Cl. 117-113 5 Claims ABSTRACT OF THE DISCLOSURE Elongated articles such as certain wire and tubing products are commonly provided with a protective coating by immersion in a liquid bath of suitable coating material. In a prefered embodiment of the subject invention the coating bath is stirred so as to establish a vortex at its surface. The elongated article is immersed in the bath and then removed therefrom in a vertical direction at the apex of the vortex. The motion of liquid in the bath at the vortex tends to form and maintain a coextensive coating of uniform thickness on the upwardly moving article.

This invention relates to methods of coating elongated articles by immersion in a liquid bath of coating material. More specifically the invention relates to a method of stirring the coating bath and withdrawing the elongated article therefrom so that a coextensive coating layer of uniform thickness is formed.

It is a common practice to coat elongated articles such as wire, tubing or pipe, with a protective, relatively thin layer of metal, paint, synthetic resin or the like. Frequently an elongated article is coated simply by immersion in a liquid bath of coating material. In processes of this type, particularly those which are employed for production purposes, the wire, tubing or other elongated article is processed at a relatively high rate of speed. Therefore, as the article is removed from the coating bath it is usually necessary to provide means for removing excess coating material adhering to the surface so that a coating layer of substantially uniform thickness is formed. Devices for this purpose are generally located so as to operate on the elongated article just as it emerges from the coating bath. Mechanical wipers which wipe or skim off adherent excess coating material by direct contact with the coating have been employed for the purposes of producing a smooth coating and of conserving coating material. Air jets, or other suitable gas jets, have been employed to wipe or blow off excess coating material for the same reasons. However, it has been found that if a rapidly moving elongated article such as tubing is not properly centered within the mechanical skimming device, or between air jets, a coating of uniform thickness is not obtained. In fact, if the rapidly moving tubing becomes only slightly misaligned the freshly applied coating may be completely scraped or blown from portions of the surface thereby exposing the underlying article to corrosion, lack of insulation or other undesirable consequences.

Accordingly, it is an object of the present invention to provide a method of coating elongated articles which eliminates the need for contact of the newly formed coating with skimming devices outside of the bath and at the same time permits the formation of a coextensive uniform coating on the surface of the article.

It is a more specific object of the present invention to provide, in combination, a method of stirring a bath of liquid coating material and of removing an immersed elongated article therefrom so that the bath itself tends Patented Nov. 10, 1970 to maintain and control the thickness of the coating layer.

It is a further object of the present invention to provide means of applying a coating on an elongated article by immersion in and subsequent withdrawal from a liquid bath of suitable coating composition in which method the coating cannot be scraped or blown from portions of the surface of the article to leave exposed areas.

In accordance with a preferred embodiment of our invention, these and other objects are accomplished by providing a liquid bath of suitable coating material and subjecting the bath to a stirring force which creates a vortex in the surface thereof. An elongated article, such as for example steel brake tubing, is immersed in the bath until wetted by the coating material. The lead-tin alloy known as terne metal is commonly used to coat steel brake tubing. The tubing is removed from the bath in a vertical direction at the apex of the vortex. We have found that the direction of movement of the stirred coating liquid at this point in the bath is such that shearing forces are exerted on any superfluous, weakly adherent molten metal present on the tubing at the tubingbath interface. These shearing forces remove such material from the adherent coating layer to leave a continuous, coextensive coating of substantially uniform thickness on the emerging tubing. Thus, in general, a bath of coating material is continuously stirred to create a vortex at one point of the surface thereof. Tubing, wire or the like is preferably continuously and rapidly passed through the bath in a suitable manner and up through the apex of the vortex to provide a uniform coating without restoring to supplemental mechanical skimming devices or gas jets to smooth out the coating film by removing superfluous material. While it is preferred that the tubing be disposed so that it passes through the apex of the vortex upon emerging from the bath, slight temporary misalignments may occur without creating extremely thin sections of coating or resulting in a complete absence of coating in localized portions of the tubing. Normally the worst thing that happens as a result of misalignment is that the coating is rough and somewhat thicker under these circumstances.

Other objects and advantages will become more apparent from a detailed description of our process in which reference will be made to the attached drawings.

FIG. 1 is an elevational view depicting a portion of a bath of molten terne metal and suitable stirring apparatus for employment in accordance with our invention; and

FIG. 2 is a somewhat enlarged view of a portion of FIG. 1 depicting the stirring apparatus, the vortex in the surface of the terne metal and the brake tubing emerging in a vertical direction from the surface thereof.

While our method is applicable to the coating of different types of articles with a number of different coating materials it may be described in terms of the continuous coating of steel brake tubing with terne alloy which is comprised nominally by weight of lead and 20% tin. Terne coated steel brake tubing is in common use throughout this country on automobiles.

Referring now to FIG. 1, a molten liquid terne metal bath 10 is confined in a suitable container 12. Steel brake tubing is preferably continuously moved toward and immersed in the terne metal bath from a direction behind and in a plane normal to the view of FIGS. 1 and 2. The tubing passes under pulley wheel 14 and is shown at 16 still immersed in the bath but now moving upwardly in a vertical direction. Disposed above bath 10 and pulley wheel 14 and extending into the bath is a stirrer assembly 18. Stirrer assembly 18 comprises a vertically disposed tube 20 supported in housing 22 with two flange bearings 24. Tube 20 is rotatable about its longitudinal axis and is driven by means of electric motor 26, pulleys 28 and 30 and V-belt 32. A steel tube 34 of sufficient inside diameter to snugly fit over tube 20 is fitted thereon and rotatable therewith. The lower end of steel tube 34 is immersed in the molten bath 10. Tube 34 has vanes 36 at its lower end for stirring the molten metal. The combination of tube 20, steel tube 34 and vanes 36 provide a rotatable hollow stirring device 38. Steel tubing 16 is drawn, by

-means not shown, upwardly and generally at the center of stirring device 38. Thus, steel brake tubing 16 is first immersed in the bath 10, wetted by the molten terne metal and then removed from the bath in a vertical direction through the center of hollow stirrer 38. By rapidly rotating hollow stirrer 38 a vortex 40 is created at the surface of terne metal bath 10. Since bath may be relatively large in volume, a housing 46 concentric with stirrer 38 and extending into the bath is employed to partially confine the portion of the molten alloy within which the vortex 40 is to be formed. Tubing I6 is aligned so as to emerge from the bath at the apex 42 of vortex 40 and a coextensive terne metal coating 44 of substantially uniform thickness is formed.

The stirring force which creates the vortex 40 causes the liquid in the bath 10 at the apex 42 to flow away from tubing 16 emerging from the bath 10 at that point. The bath metal in the vicinity of the vortex 40 is moving substantially normal to the direction of the tubing 16. This localized flow of the molten terne metal exerts shear forces on that liquid terne metal adhering to the surface of the tubing and thereby limits the thickness of the coating. Since the shear forces are substantially equal in all directions about the circumference of the tubing, the thickness of the coating is normally uniform in this area. It is noted that in accordance with our method the thickness of the coating 44 is controlled by the stirring of the bath as described and the rate of advancement of the tubing. No external mechanical wiping devices are required or employed. Moreover, coating 44 is partially characterized by the absence of thin portions and discontinuities which would expose the steel tubing to corrosive elements.

Experiments have been conducted with an apparatus as depicted in the drawings in which the stirrer was rotated at a fairly uniform rate of 240 r.p.m. Brake line tubing was moved into the molten terne metal bath and up through the hollow stirrer 38 and apex 42 of vortex 40 at a rate of 65 feet per minute. A section of the coated tubing was then examined. Under the conditions described the average coating weight was found to be 1.08 oz. per square foot. The minimum coating thickness was found to be 0.00005 inch, the maximum thickness was found to be 0.0025 inch and the average thickness was 0.0010 inch. The tubing was exposed to a chemical agent for the purpose of detecting the presence of uncoated areas. No uncoated areas were found; the process had produced a truly coextensive coating.

Another experiment was performed in which the tubing was passed through the bath and up through the hollow stirrer and vortex at a rate of 75 feet per minute, the stirrer being rotated at 24 r.p.m. After a substantial length of tubing had been coated with terne metal it was examined and found that the average weight of coating was 1.33 oz. per square foot. The minimum coating thickness was 0.0002 inch, the maximum thickness was 0.0035 inch and the average thickness was 0.0013 inch. Again the tubing was exposed to a chemical agent to detect the presence of uncoated areas, but none were found.

A further experiment was performed in which brake tubing was advanced through the bath, as described, at a rate of 85 feet per minute. When an appreciable length of tubing had been so coated it was examined as above. It was found that the average coating weight was 1.36 oz. per square foot. The minimum coating thickness was 0.00005 inch, the maximum 0.0035 inch and the average thickness was 0.0014 inch. No uncoated areas were found on the tubing. Thus, despite a substantial change in the Cir speed at which the brake tubing was advanced through the bath an acceptable coating which was coextensive and reasonably uniform in thickness was produced. The term uniform thickness as employed herein is intended to imply that the coating contained few, if any, drops, runs or other readily observable irregularities in the surface of the coating. Moreover, coated articles produced by our method contain no bare or extremely thin areas which may expose the substrate to corrosive or other undesirable conditions.

For purposes of comparison, random samples of brake tubing were examined which had been produced by immersing steel tubing in terne metal and then subjecting the coated tubing to air jets for the purpose of removing excess metal. It was found that the average weights of coating material on these samples were in the range of 0.250.65 oz. per square foot. The minimum coating thickness noted was less than 0.00001 inch while the maximum thickness was about 0.004 inch. The average coating thicknesses of the samples tested were in the range of 0.002 inch to 0.0007 inch. Occasional uncoated areas were detected when the tubing was subjected to a chemical agent employed for this purpose. As might be expected from the presence of surface irregularities on the air wiped or mechanically wiped terne coated tubes, the corrosion rates of these coated tubes were higher than the rates obtained in the same tests conducted on the tubes produced in accordance with our invention.

It will be appreciated that depending on the nature of the coating material and the article to be coated it may be necessary to experimentally determine the most desirable rate of stirring and rate of advancement of the elongated article through the bath and upwardly through the apex of the vortex in the surface of the coating bath. It is expected that, in general, the higher the rate of stirring the greater the shearing forces tending to remove superfluous coating material which, in general, would permit a higher rate of advancement of the article to be coated to maintain the coating at a predetermined thickness range.

In general, any alongated article which is relatively small and fairly circular in cross section such as tubing, wire and small diameter pipe, may be coated by our method. The coating material itself can be molten metal, ceramic materials, synthetic resins and the like. When necessary, particulate coating materials may be immersed, dissolved or dispersed in a suitable liquid vehicle. The selection of the proper combination of article to be coated and coating material is deemed to be within the skill of one familiar with the materials involved and results sought to be obtained, Our method will be useful in applications in which elongated articles are to be coated by immersion in liquid bath which can be suitably stirred to produce a vortex therein.

In the description of our invention a mechanical stirring device was employed in which the stirring means was in the form of a hollow tube. It will readily be recognized that other stirring means could be employed. For example in the case of molten metals electromagnetic stirring could be employed to introduce a vortex in the liquid.

Accordingly, While our invention has been described in terms of specific embodiments thereof it will be appreciated that other forms can readily be adapted by those skilled in the art, therefore, our invention is to be considered limited only by the scope of the appended claims.

We claim:

1. A method of coating an elongated article of relatively small and generally circular transverse cross section comprising providing a liquid bath of coating material, subjecting said bath to a force which stirs such liquid so as to create a vortex therein, immersing at least a portion of a said elongated article in said bath until the surface thereof has been wetted by said liquid and removing said article from said bath in a direction upwardly through the apex of said vortex whereby a coextensive coating is formed on said article of substantially uniform thickness.

2. A method of coating an elongated article of relatively small and generally circular transverse cross section comprising providing a liquid bath of coating material, stirring said bath so as to create a vortex at the surface thereof, immersing said elongated article in said bath until the surface thereof has been wetted by said liquid and removing said article from said bath in a direction upwardly through the apex of said vortex, the movement of the liquid in said bath in the vicinity of said vortex subjecting the liquid tending to adhere to said article to shear forces whereby a coextensive coating of substantially uniform thickness is formed on said article.

3. A method of coating an elongated metal product of relatively small transverse cross section with a lower melting metal comprising preparing a molten bath of said lower melting metal, subjecting said bath to a stirring force to create a vortex at the surface thereof, immersing at least a portion of said elongated product in said bath until the surface of said article has been wetted by said molten metal, and removing said wetted article from said bath vertically at the apex of said vortex whereby the moving liquid in said bath at said vortex exerts a force on excess liquid tending to adhere to the surface of said article such that said article is provided with a coexten sive coating of substantially uniform thickness.

4. A method of terne coating steel tubing comprising preparing a molten bath of terne alloy, stirring said molten bath to produce a vortex at the surface thereof, passing said steel tubing into said bath and removing it therefrom vertically at the apex of said vortex so that the bath metal in the vicinity of said vortex is moving normal to the direction of said tubing at said apex and shear stresses are thereby induced at the molten metaltubing interface to remove excess coating metal leaving a coextensive layer of teme metal on the surface of said tubing of substantially uniform thickness,

5. A method of coating an elongated article of relatively small and generally circular transverse cross section comprising providing a liquid bath of coating material, mechanically spinning the liquid in at least a portion of said bath in a rotational fashion thereby creating a vortex at the surface of the liquid, immersing said elongated article in said bath until the surface thereof has been wetted by said liquid and removing said article from said bath in a direction upwardly through the apex of said vortex, the movement of the liquid in said bath in the vicinity of said vortex subjecting the liquid tending to adhere to said article to shear forces whereby a coextensive cvoating of substantially uniform thickness is formed on said article.

References Cited UNITED STATES PATENTS 1,332,585 3 /1920 White. 2,325,156 7/ 1943 Whitfield. 2,489,456 11/1949 Liebel l17-115 2,683,099 7/1954 Hahn 117-114 3,410,534 11/1968 Wyczalek ll7-l14X ALFRED L. LEAVITT, Primary Examiner I. R. BATTEN, JR., Assistant Examiner US. Cl. X.R.

l17ll4, 115, 128; 118-612 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 539, 385 Dated November 10, 1970 Inventor(s) Joseph E. Hunter and William K. Miller It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 32, "restoring" should be resortin Column 3, line 38, "partially" should be particularly ---1 line 59, "24 r.p.m." should be 240 r.p.m. Column 4, line 41., alongated should be elongated Column 6, line 18, "cvoating" should be coating S'Lfi'li'rli) 8ND SEALED M9 1971 (QEAL) mm SGHUYIAR RI p I Mum, commissioner of Patna Arresting Office! 

